1. Field of the Invention
The present invention relates generally to immunology and to methods for the diagnosis and treatment of cancer. Disclosed are methods for characterizing distinct sub-sets of tumor cells from within a tumor population, and novel methods of cancer treatment based upon the nature of the cell types identified. In particular, the present invention relates to the identification of tumor cells that are in cell cycle arrest, and to methods for either the elimination or maintenance of such cell cycle arrested populations.
2. Description of the Related Art
The relationship between malignant tumors and their host can be divided into three broad categories: first, continuous growth of the tumor resulting in death of the host; second, complete eradication of tumor cells by natural mechanisms or therapy; and third, a prolonged quiescent state in which tumor cells are present but tumor growth is not clinically apparent. Viable tumor cells which remain present in their host, under some form of growth restraint, can be said to be `arrested` or to exhibit the phenomenon of tumor dormancy. The existence of such dormant cells can be demonstrated by the eventual growth of a tumor in the host, or, in animal models, by the transfer of tumor to naive recipients.
In human subjects, neoplastic cells are believed to escape from a primary tumor very early in its development. The fate of these escaping neoplastic cells will usually determine the prognosis of the patient. In many cases, tumor cells may remain clinically undetectable for several years. However, this type of micrometastasis in which growth does not occur for long periods of time, frequently ends with the re-emergence of the tumor.
There are many examples of tumor recurrence in humans, both at the primary site or at a distant, metastatic site, many years after the removal of the primary neoplasm (Wheelock et al., 1981). Clinical examples of this include, for example, melanoma and breast carcinoma, where the period of clinical latency can be a decade or more. Indeed, recurrence of breast cancer at the site of surgical incision has been observed after an interval of 50 years (Meltzer, 1990). The phenomenon of waxing and waning of B cell tumors, in the absence of any form of therapy, has also been observed (Krikorian et al., 1980).
An analysis of relapse rates in acute myelogenous leukemia (AML) patients after complete response indicates that relapse can be divided into two phases, an early phase and a late phase, with different relapse rates suggesting that they occur by two different mechanisms. One possible explanation for late relapse is that treatment eliminated the majority of cancer cells, but also altered a small number of cells so that they became resistant and eventually expanded resulting in relapse. If this were the case, the re-emergent tumor would be resistant to the same chemotherapeutic agents. However, this is usually not the case. An alternative explanation is that at the time of therapeutic intervention some of the cancer cells were in a quiescent state. Since most therapeutic agents are effective against dividing cells, the dormant cancer cells would be relatively resistant to treatment, and relapse would be a consequence of these cells subsequently proliferating.
The mechanisms that underlie induction, maintenance, and recurrence of cancers are clearly of great medical importance. Despite such clinical implications, and the fundamental insights that the study of distinct tumor cell populations might give to the biology of cancer and growth control of cells, the topic has received surprisingly little attention, with probably less than 50 experimental studies being published in the last half century. As such, there is no data which currently relates in vitro observations to the more complex situation in vivo. Neither is it known whether dormant cancer cells do not divide, or whether they continue to divide and are killed at the same rate.
A variety of mechanisms, both immunological and non-immunological, have been proposed to be responsible for the long-term behavior of tumor cells. These mechanisms may operate by restraining the tumor cells, or by killing the cells at a rate equal to their division. The eventual escape of tumor cells from a resting, or dormant, stage might be a consequence either of alterations in the host animal, or in the tumor cell, or caused by some interaction between the host and the malignant cells. For example changes in the host's immunological status could allow the tumor to escape from immune control, as could the generation of antigen-negative variant cells. In addition, the vascularization of the tumor, or the movement of neoplastic cells into a site more favorable for their growth, could lead to the breakdown of quiescence (Wheelock et al., 1981).
It is generally believed that cellular immune mechanisms largely control whether tumor cells enter a resting stage as opposed to a growth and reproductive stage. The initiation of dormancy is thought to be a predominantly cytotoxic T-cell mediated process (Weinhold et al., 1979), with longer-term maintenance of this state requiring the further recruitment of macrophages (Robinson & Wheelock, 1981). The re-emergence of tumor cells after a resting period has been reported to be associated with the appearance of macrophages which secrete prostaglandin E.sub.2 (Liu et al., 1986), an immunosuppressive factor, as well as, in some cases, selection of variants with reduced antigenicity (Trainer & Wheelock, 1984).
The identification, isolation and characterization of distinct cell types from within a tumor cell population would likely lead to significant advances in our understanding of cancer. Information regarding the characteristics of dormant or quiescent tumor cells would be of great value clinically, creating new possibilities for therapeutic intervention. The development of PCR technology which has facilitated detection of minimal residual diseases in humans, adds impetus to identify and characterize different tumor cell types in order to develop new treatment strategies for cancer patients.